The present invention relates generally to a coaxial cable particularly suited for use in high voltage RF applications.
A need exists for an improved coaxial cable for interconnecting a transmitter which generates high frequency RF power (in the order of 100 KW at peak RF voltages approximating 40 KV) to a suitable antenna or to a dummy load. Such a coaxial cable is susceptible to a corona discharge at the interface of the center conductor and the surrounding primary insulation material. A corona discharge can also occur between the primary insulation material and the braided outer conductor or shield of the coaxial cable, resulting in the loss of power and excessive heating of the cable at higher frequencies of operation.
Such corona discharges result mainly from the ionization of air in voids or cavities which exist between the primary insulation material at its interfaces with the center conductor and with the braided outer shield. The elimination of corona discharge in any high voltage cable therefore requires that either the voltage stress be controlled to a level below corona onset, or the elimination of air at each high voltage interface. Since in high voltage applications, the cable size would generally be too large if the voltage stresses were kept below corona onset, most high voltage coaxial cables are designed using the latter approach.
One technique presently used in the design of high voltage coaxial cable involves the extrusion of a semiconductor material over the center conductor such that gaps are not formed therebetween. The semiconductor material and the insulation material are generally co-extruded onto the center conductor and after curing, there should be no gaps at their interface.
When a high voltage is applied to the center conductor, the semiconductor material, which is in electrical contact with the center conductor, is also at the same voltage. Since there is no potential difference between the center conductor and the semiconductor material, any air that might be trapped at that interface is not stressed. The high voltage interface between the semiconductor material and its surrounding primary insulation material, is also a substantially air-free interface and the stresses can be higher without creating corona discharge. As previously mentioned, this technique is utilized in many high voltage cables.
When the voltage applied to a coaxial cable is high enough, corona can also exist at the outer boundary of the primary insulation material and the braided metallic shield formed over the primary insulation material, due to the entrapment of air therebetween. One presently known method of eliminating the air at the primary insulation/outer braid interface is to either extrude a second semiconductor layer over the primary insulation or to paint on a carbon filled ink and wrap over the ink a layer of carbon filled fabric tape. The same principle as described earlier applies here, in that the semiconductor layer and braided shield are now at the same voltage and any air at that boundary is not stressed.
Another method of avoiding corona discharge in electrical cable is mentioned in U.S. Pat. No. 3,259,688 issued to Allen N. Towne et al on July 5, 1966. It is mentioned therein that tapes formed of elastomeric material or impregnated with semiconductive materials such as graphite, carbon black, and the like may produce voids at points of overlapping, fostering the production of corona and defeating the purpose of the electrically conductive grading material.
Towne et al therefore propose the use of a composite insulation comprising a first taped or sheet layer of semiconducting material comprising copolymers of polyethylene and mono-unsaturated materials along with a ground insulation of extruded polyethylene. Another layer of polyethylene copolymer material may surround the primary insulation over which a metallic shield is wrapped.
While the aforementioned solutions to the corona problem at the center conductor/primary insulator interface are generally acceptable, the requirement for a second such extruded semiconductor layer or impregnated tape layer at the primary insulation/outer shield interface results in a coaxial cable that is much more difficult and expensive to manufacture, requiring tightly controlled manufacturing processes. Furthermore, the quality over life of the cable will be greatly reduced and the cable will be limited to use in environments which do not have low temperatures and applications which do not require high vibration or flexure of the cable.